In physicochemical appliances, for example, nuclear magnetic resonance (NMR) appliances, that use a superconducting magnet to create a uniform magnetic field and measure physical and chemical characteristics of a substance in the magnetic field, liquid helium is conventionally used as a cryogen to cool the superconducting magnet (to about -270.degree. C.). The liquid helium is stored in a liquid helium storage tank that is surrounded by a liquid nitrogen storage tank to reduce heat penetration into the liquid helium storage tank and reduce evaporation of the liquid helium.
Such physicochemical appliances are susceptible of certain disadvantages and drawbacks in that they consume significant amounts of liquid nitrogen as compared with the consumption of liquid helium, thereby requiring much expense in time and labor for replenishment of the liquid nitrogen. Japanese Patent Application Laid-Open No. Hei 8-327171 proposes a solution in an attempt to address this problem. The system described in this document is illustrated in FIG. 9.
As seen in FIG. 9, a vacuum insulation vessel 51 of an NMR apparatus contains a liquid helium storage tank 53 filled with liquid helium. A superconducting magnet 52 is located in the liquid helium in the liquid helium storage tank 53. Surrounding the outer periphery of the liquid helium storage tank 53 is a liquid nitrogen storage tank 54 filled with liquid nitrogen.
A re-liquefaction vessel 55 includes a vacuum insulation structure in which a re-liquefied gas pool portion 56 is formed. A cryogenic refrigerator is mainly made up of a cold head 58 and a compressor 59. A low temperature generating portion 60 of the cold head 58 protrudes toward or into the re-liquefied gas pool portion 56.
The vacuum insulation vessel 51 is connected to the re-liquefaction vessel 55 by a flexible pipe 61. The liquid nitrogen storage tank 54 of the vacuum insulation vessel 51 thus communicates with the re-liquefied gas pool portion 56 of the re-liquefaction vessel 55.
In this construction, nitrogen gas formed in the liquid nitrogen storage tank 54 of the vacuum insulation vessel 51 is conveyed by the flexible pipe 61 into the re-liquefaction vessel 55. In the re-liquefaction vessel 55, nitrogen gas is re-liquefied by the low temperature generating unit 60 of the cryogenic refrigerator, and re-liquefied gas (liquid nitrogen) is pooled in the re-liquefied gas pool portion 56 formed in the lower portion of the re-liquefaction vessel 55. Overflow of the re-liquefied gas (liquid nitrogen) from the re-liquefied gas pool portion 56 flows down through the flexible pipe 61 and thus returns to the liquid nitrogen storage tank 54. Therefore, the consumption of liquefied gas is reduced and the time period for liquefied gas replenishment is lengthened.
In this conventional technology, however, the cooling system for cooling and re-liquefying the nitrogen produces significant mechanical vibrations. Thus, the NMR apparatus and the re-liquefaction vessel are disposed apart from each other, with the flexible pipe connecting the two, in order to prevent mechanical vibrations produced by the refrigerator from being transmitted to the NMR apparatus. In this way, reductions in the measurement precision of the NMR apparatus can be prevented. Unfortunately, this results in a rather complicated construction having a relatively large size.
In light of the foregoing, a need exists for a superconducting magnet system which is able to effectively cool the superconducting magnet, yet which is relatively compact in size and does not require the same complicated construction as other known systems.